arXiv subjects

Sebin J. Sebastian

Publications and source records attributed to Sebin J. Sebastian.

9 recordsLinked to original sources

arXiv2026

Ground-state properties of the $S=3/2$ anisotropic triangular lattice antiferromagnet Na$_3$Cr(PO$_4$)$_2$

We report the crystal structure and magnetic properties of a $S=3/2$ anisotropic triangular lattice compound Na$_3$Cr(PO$_4$)$_2$ employing single-crystal and powder x-ray diffraction, magnetization, heat capacity, and $^{31}$P nuclear magnetic resonance (NMR) experiments, supported by the band structure calculations. Magnetic susceptibility exhibits a broad maximum around 3.5 K, indicating the presence of a short-range antiferromagnetic order, typical of a low-dimensional spin system. Magnetization and heat capacity manifest an antiferromagnetic long-range ordering at around $T_{\rm N} \simeq 2.6$ K. This was further confirmed by the drastic NMR line broadening and a peak in the nuclear spin-lattice and spin-spin relaxation rates. The isothermal magnetization data exhibit a field-induced spin-flop transition at around $\mu_0H_{\rm SF} \simeq 1.7$ T reminiscent of an anisotropic two-dimensional magnet, before saturating above $\mu_0H_{\rm sat} \simeq 4.5$ T. The saturation field was further upheld by the field-dependent NMR relaxation measurements at low temperatures. The $^{31}$P NMR spectral shape confirms the commensurate antiferromagnetic nature of the ordering below $T_{\rm N}$. \textit{Ab initio} calculations reveal a significant deformation of the triangular spin lattice, resulting in triangles with two antiferromagnetic couplings of similar strength and a much weaker coupling along the third side of the triangle.

cond-mat.mtrl-sci↗

arXiv2025

Static and dynamic properties of the frustrated spin-1/2 depleted-kagome antiferromagnet Cu$_7$(TeO$_3$)$_2$(SO$_4$)$_2$(OH)$_6$

The structural and magnetic properties of the two-dimensional spin-$1/2$ depleted-kagome compound Cu$_7$(TeO$_3$)$_2$(SO$_4$)$_2$(OH)$_6$ are investigated using x-ray diffraction, magnetization, heat capacity, and $^1$H Nuclear Magnetic Resonance (NMR) measurements. From the analysis of magnetic susceptibility, we found a large Curie-Weiss temperature [$\theta_{\rm CW} = -50(2)$ K] and the co-existence of antiferromagnetic and ferromagnetic interactions. The value of $\theta_{\rm CW}$ gives an estimate of the average nearest-neighbour antiferromagnetic interaction of $J/k_{\rm B} \simeq 66$ K. The NMR relaxation rates ($1/T_1$ and $1/T_2$) exhibit a peak, providing evidence for a magnetic long-range order at $T^*\simeq 4$ K which appears to be canted antiferromagnetic type. Heat capacity also features a broad maximum at $T^*$ that moves towards higher temperatures with increasing magnetic field, reflecting defect induced Schottky anomaly. The frustration parameter $f_r = \lvert \theta_{\rm CW} \lvert/{T^{*}}\simeq 12.5$ renders the compound a highly frustrated low-dimensional magnet.

cond-mat.mtrl-sci↗

arXiv2025

Frustration driven magnetic correlations in the spin-$5/2$ triangular lattice antiferromagnet RbFe(HPO$_{3}$)$_{2}$

A detailed study of the structural and magnetic properties of a spin-$5/2$ triangular lattice antiferromagnet RbFe(HPO$_{3}$)$_{2}$ is presented using x-ray diffraction, magnetization, heat capacity, and $^{31}$P nuclear magnetic resonance (NMR) experiments on a polycrystalline sample. The crystal structure features an equilateral triangular lattice of Fe$^{3+}$ ions. The thermodynamic measurements reveal the onset of a magnetic long-range order at $T_{\rm N1} \simeq 7.8$ K in zero-field, followed by another low temperature field induced ordering at $T_{\rm N2}$ in higher fields. The transition at $T_{\rm N1}$ is further confirmed from the NMR spin lattice relaxation measurements. The value of the frustration ratio ($f \simeq 7$) implies moderate spin frustration in the compound. The $^{31}$P NMR spectra exhibit two distinct spectral lines corresponding to two inequivalent phosphorus sites (P1 and P2), consistent with the crystal structure. The P1 site is strongly coupled with an isotropic hyperfine coupling of $A_{\rm hf}^{\rm iso} = 0.55(2)$ T/$\mu_{\rm B}$ while the P2 site is weakly coupled with $A_{\rm hf}^{\rm iso} = 0.25(3)$ T/$\mu_{\rm B}$ with the Fe$^{3+}$ ions. The magnetic susceptibility and NMR shift data are described well assuming a spin-$5/2$ isotropic triangular lattice antiferromagnetic model with an average exchange coupling of $J/k_{\rm B} = 2.8(2)$ K. Below $T_{\rm N1}$, the spectra evolve into a nearly rectangular powder pattern, indicating a commensurate antiferromagnetic type order. The $^{31}$P spin-lattice relaxation rate well below $T_{\rm N1}$ follows a $T^3$ temperature dependence, implying a two-magnon Raman scattering mechanism in the ordered state. Three well-defined phase regimes are clearly ascertained in the $H-T$ phase diagram, reflecting a weak magnetic anisotropy in the compound.

cond-mat.mtrl-sci↗

arXiv2025

Short-range Spin Freezing State in the Double Trillium Lattice Spin-Liquid Candidate KSrFe$_2$(PO$_4$)$_3$ Revealed via $^{31}$P NMR

A comprehensive $^{31}$P nuclear magnetic resonance (NMR) study, combined with thermodynamic measurements and first-principle band-structure calculations, has been conducted to explore the ground state of the $S = 5/2$ double trillium lattice antiferromagnet KSrFe$_2$(PO$_4$)$_3$. Our experimental results indicate that the magnetic ground state is neither a conventional three-dimensional (3D) long-range order (LRO) nor a pure gapless spin-liquid state, as conjectured previously [Boya et al., APL Mater. 10, 101103 (2022)]. Specifically, the observation of a nearly field-independent NMR linewidth below $T^{*}$ = (3.5 $\pm$ 0.4) K, and a significant enhancement of spin-spin relaxation rate $1/T_2$ below $2T^{*}$ (where $T^{*}$ is the characteristic temperature identified from the magnetic susceptibility), indicate a complex magnetic ground state where spin freezing coexists with persistent dynamics. Furthermore, we argue that the lack of magnetic LRO and the persistence of strong magnetic fluctuations in KSrFe$_2$(PO$_4$)$_3$ are unlikely to originate from intersite K/Sr disorder, rather arise due to intrinsic magnetic frustration. Our findings position KSrFe$_2$(PO$_4$)$_3$ into a broader family of geometrically frustrated magnets characterized by coexisting spin freezing and pronounced antiferromagnetic fluctuations, marking it as a promising platform for investigating exotic phenomena in 3D frustrated magnets.

cond-mat.str-el↗

arXiv2025

Spin fluctuations, absence of magnetic order, and crystal electric field studies in the Yb$^{3+}$-based triangular lattice antiferromagnet Rb$_3$Yb(VO$_4$)$_2$

We report a comprehensive experimental investigation of the structural, thermodynamic, static, and dynamic properties of a triangular lattice antiferromagnet Rb$_3$Yb(VO$_4$)$_2$. Through the analysis of magnetic susceptibility, magnetization, and specific heat, complemented by crystal electric field (CEF) calculations, we confirm the Kramers' doublet with effective spin $J_{\rm{eff}}=1/2$ ground state. Magnetic susceptibility and isothermal magnetization analysis reveal a weak antiferromagnetic interaction among the $J_{\rm{eff}}=1/2$ spins, characterized by a small Curie-Weiss temperature ($\theta_{\text{CW}}^{\text{LT}}\simeq-0.26$ K) or a reduced exchange coupling ($J/k_{\rm B} \simeq 0.18$ K). The $^{51}$V NMR spectra and spin-lattice relaxation rate ($1/T_1$) show no evidence of magnetic long-range-order down to 1.6 K but reflect strong influence of CEF excitations in the intermediate temperatures. At low temperatures, $1/T_1(T)$ shows pronounced frequency dependence and $1/T_1$ vs field in different temperatures follows the scaling behaviour, highlighting the role of paramagnetic fluctuations. The CEF calculations using the point charge approximation divulge a large energy gap ($\sim 18.61$ meV) between the lowest and second lowest energy doublets, further establishing Kramers' doublet as the ground state. Our calculations also reproduce the experimental magnetization and specific heat data and indicate an in-plane magnetic anisotropy. These findings position Rb$_3$Yb(VO$_4$)$_2$ as an ideal and disorder-free candidate to explore intrinsic quantum fluctuations and possible quantum spin-liquid physics in a Yb$^{3+}$-based triangular lattice antiferromagnet.

cond-mat.mtrl-sci↗

arXiv2023

Disordered ground state in the spin-orbit coupled $J_{\rm eff}= 1/2$ cobalt-based metal-organic framework magnet with orthogonal spin dimers

We present the magnetic properties of a strongly spin-orbit coupled quantum dimer magnet based on Co$^{2+}$. The metal-organic framework compound Co$_2$(BDC)$_2$(DPTTZ)$_2$$\cdot$DMF features Co$^{2+}$ dimers arranged nearly orthogonal to each other, similar to the Shastry-Sutherland lattice. Our assessment based on the magnetization and heat capacity experiments reveals that the magnetic properties at low temperatures can be described by an effective $J_{\rm eff} = 1/2$ Kramers doublet and the ground state is a singlet with a tiny spin gap. Although the magnetic susceptibility could be analyzed in terms of the interacting dimer model with an isotropic intradimer coupling $J_0/k_{\rm B} \simeq 7.6$ K, this model fails to reproduce the shape of magnetization isotherm and heat capacity data. A model of isolated spin dimers with the anisotropic exchange couplings $J_{xy} \simeq 3.5$ K and $J_{z} \simeq 11$ K provides an adequate description to the magnetic susceptibility, magnetization isotherm, and heat capacity data at low temperatures. Interestingly, no field-induced quantum phase phase is detected down to 100~mK around the critical field of gap closing, suggesting the absence of Bose-Einstein condensation of triplons and establishing isolated dimers with a negligible interdimer coupling.

cond-mat.str-el↗

arXiv2022

Static and dynamic magnetic properties of the spin-5/2 triangle lattice antiferromagnet Na3Fe(PO4)2 studied by 31P NMR

$^{31}$P nuclear magnetic resonance (NMR) measurements have been carried out to investigate the magnetic properties and spin dynamics of Fe$^{3+}$ ($S$ = 5/2) spins in the two-dimensional triangular lattice (TL) compound Na$_3$Fe(PO$_4$)$_2$. The temperature ($T$) dependence of nuclear spin-lattice relaxation rates ($1/T_1$) shows a clear peak around N\'eel temperature, $T_{\rm N} = 10.9$~K, corresponding to an antiferromagnetic (AFM) transition. From the temparature dependence of NMR shift ($K$) above $T_{\rm N}$, an exchange coupling between Fe$^{3+}$ spins was estimated to be $J/k_{\rm B}\simeq 1.9$~K using the spin-5/2 Heisenberg isotropic-TL model. The temperature dependence of $1/T_1T$ divided by the magnetic susceptibility ($\chi$), $1/T_1T\chi$, above $T_{\rm N}$ proves the AFM nature of spin fluctuations below $\sim$ 50 K in the paramagnetic state. In the magnetically ordered state below $T_{\rm N}$, the characteristic rectangular shape of the NMR spectra is observed, indicative of a commensurate AFM state in its ground state. The strong temperature dependence of 1/$T_1$ in the AFM state is well explained by the two-magnon (Raman) process of the spin waves in a 3D antiferromagnet with a spin-anisotropy energy gap of 5.7 K. The temperature dependence of sublattice magnetization is also well reproduced by the spin waves. Those results indicate that the magnetically ordered state of Na$_3$Fe(PO$_4$)$_2$ is a conventional 3D AFM state, and no obvious spin frustration effects were detected in its ground state.

cond-mat.str-el↗

arXiv2022

Collinear antiferromagnetic order in spin-$\frac52$ triangle lattice antiferromagnet Na$_3$Fe(PO$_4$)$_2$

We set forth the structural and magnetic properties of the frustrated spin-$5/2$ triangle lattice antiferromagnet Na$_3$Fe(PO$_4$)$_2$ examined via x-ray diffraction, magnetization, heat capacity, and neutron diffraction measurements on the polycrystalline sample. No structural distortion was detected from the temperature-dependant x-ray diffraction down to 12.5 K, except a systematic lattice contraction. The magnetic susceptibility at high temperatures agrees well with the high-temperature series expansion for a spin-$5/2$ isotropic triangular lattice antiferromagnet with an average exchange coupling of $J/k_{\rm B} \simeq 1.8$ K rather than a one-dimensional spin-$5/2$ chain model. This value of the exchange coupling is consistently reproduced by the saturation field of the pulse field magnetization data. It undergoes a magnetic long-range-order at $T_{\rm N} \simeq 10.4$ K. Neutron diffraction experiments elucidate a collinear antiferromagnetic ordering below $T_{\rm N}$ with the propagation vector $k = (1,0,0)$. An intermediate value of frustration ratio ($f \simeq 3.6$) reflects moderate frustration in the compound which is corroborated by a reduced ordered magnetic moment of $\sim 1.52$ $\mu_{\rm B}$ at 1.6 K, compared to its classical value ($5 \mu_{\rm B}$). Magnetic isotherms exhibit a change of slope envisaging a field induced spin-flop transition at $H_{\rm SF}\simeq3.2$ T. The magnetic field vs temperature phase diagram clearly unfold three distinct phase regimes, reminiscent of a frustrated magnet with in-plane (XY-type) anisotropy.

cond-mat.mtrl-sci↗

arXiv2021

Quasi-one-dimensional magnetism in the spin-$\frac12$ antiferromagnet BaNa$_{2}$Cu(VO$_{4}$)$_{2}$

We report synthesis and magnetic properties of quasi-one-dimensional spin-$\frac{1}{2}$ Heisenberg antiferromagnetic chain compound BaNa$_2$Cu(VO$_4$)$_2$. This orthovanadate has a centrosymmetric crystal structure, $C2/c$, where the magnetic Cu$^{2+}$ ions form spin chains. These chains are arranged in layers, with the chain direction changing by 62$^0$ between the two successive layers. Alternatively, the spin lattice can be viewed as anisotropic triangular layers upon taking the inter-chain interactions into consideration. Despite this potential structural complexity, temperature-dependent magnetic susceptibility, heat capacity, ESR intensity, and NMR shift agree well with the uniform spin-$1/2$ Heisenberg chain model with an intrachain coupling of $J/k_{\rm B} \simeq 5.6$ K. The saturation field obtained from the magnetic isotherm measurement consistently reproduces the value of $J/k_{\rm B}$. Further, the $^{51}$V NMR spin-lattice relaxation rate mimics the 1D character in the intermediate temperature range, whereas magnetic long-range order sets in below $T_{\rm N} \simeq 0.25$ K. The effective interchain coupling is estimated to be $J_{\perp}/k_{\rm B} \simeq 0.1$ K. The theoretical estimation of exchange couplings using band-structure calculations reciprocate our experimental findings and unambiguously establish the 1D character of the compound. Finally, the spin lattice of BaNa$_2$Cu(VO$_4$)$_2$ is compared with the chemically similar but not isostructural compound BaAg$_2$Cu(VO$_4)_2$.

cond-mat.mtrl-sci↗